76 Transactions of the 



in tlie whole ammonite was the siphnncle, a means of hydraulic 

 adjustment by which the animal regulated its ascent or descent. 

 The exact arrangement is not clearly made out, but it seems to 

 have been connected with a large sac or pericardium inside the 

 animal, which was filled with fluid, the shell being caused to sink 

 when the fluid was expelled from the pericardium into the siphuncle, 

 and rising when the fluid was again withdrawn into the sac. The 

 difl'erence between these tubes in the nautili and ammonites is, 

 that the nautili have them in the centre of the chambers and 

 ammonites against the back, sometimes causing a ridge upon that 

 part. Another interesting class of fossils is the belemnites. 

 They occur in every stratum of the secondary formations, gene- 

 rally in great profusion, but only sparsely in the great oolite at 

 Minchinhampton. They were the supporting bones of a species 

 of sepia, corresponding to the large white bones of cuttlefish 

 picked up at the present day. It is, therefore, an internal shell of 

 compoi^nd structure made up of three essential parts, — First, a 

 fibrocalcareous cone-shaped shell, terminating at its larger end in 

 a hollow ; then a thin conical horny sheath or cup from the 

 base of the fibrous cone, enlarging rapidly and extending outwards 

 to a considerable distance (this cup contained the inkbag and 

 other viscera) ; and lastly, a conical chambered shell within the 

 cup, called the phragmocone or alveolus ; it was divided by thin 

 transverse plates into air-chambers or areolae, which were per- 

 forated by a continuous siphuncle placed on the inferior margin. 



The nautili belong to Owen's order of Tetrahranchiata. Three 

 species are found at Minchinhampton. They roughly resemble 

 a disc, and are much convoluted, sometimes compressed ; the 

 whorls are contiguous, the last one generally widening at the 

 mouth so as to cover the rest ; the septa are transverse and con- 

 cave, with the perforation for the siphuncle more or less in the 

 centre. The first species is Nautilus disponsus — a large round 

 and smooth shell with the back of the whorls rounded ; the mouth 

 very large. Only one specimen has been obtained from Minchin- 

 hampton ; but four miles off", in Woodchester, it is more common. 



The smaller fossils of the neighbourhood are by far the most 

 numerous. Dr Buckland (" Bridgewater Tr.," vol. i., p. 299) has 

 remarked that in the strata from the chalk downwards, car- 

 nivorous univalves are rare, though extremely abundant in the 

 tertiary rocks, and brings forward the trachelipods as an instance. 

 However, in the uppermost portion of that part of the great 

 oolite called the planking, a bed eight or ten feet in thickness, 

 we find a numerous set of these trachelipods, and of the 141 

 species of univalves, 45 at least are carnivorous, comprised in 

 about seven genera. The extreme scarceness of the cephalopoda 



